Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the automobile internal combustion control system to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Generally, the electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble shaped electrochemical galvanic cell to determine, or sense, the relative amounts of oxygen present in the exhaust stream, an example being U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor is generally known and used throughout the automotive industry, and comprises an ionically conductive solid electrolyte material, typically yttria stabilized zirconia, a porous electrode coating on the exterior exposed to the exhaust or measuring gas and a porous electrode coating on the interior exposed to a known concentration of reference gas. The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes by the Nernst equation: E=AT ln[P.sub.1 /P.sub.2 ], where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant. Thus, the oxygen sensor senses the oxygen concentration in the exhaust gas by measuring this galvanic output voltage.
However, the solid electrolyte of such a sensor must first be heated to an elevated temperature in order to obtain an appreciable output voltage in response to the difference in the oxygen concentrations between the reference and measuring electrodes. The induced galvanic potential between electrodes and corresponding output voltage are not stable until the solid electrolyte has been heated to a given temperature. The combustion gases heat the solid electrolyte of the oxygen sensor to an operating temperature sufficient to effect galvanic stability. Effective sensor operation is therefore delayed until the combustion gases heat the sensor to a suitable temperature.
In addition, if the sensor is placed too far downstream in the exhaust pipe of an engine, especially a highly efficient engine, the sensor may not be heated to a high enough temperature during engine idle to meet sensor specifications. During these conditions, the internal combustion engine control system operates open loop, i.e., the control system does not sense the controlled parameter, air-to-fuel ratio, in order to control that parameter. It is known that a large percentage of the total emissions produced during short period of operation are produced during this period, engine warm up. In some applications, emissions control during engine warm up might be improved with an oxygen sensor which had means for rapidly heating itself to a predetermined temperature, regardless of the temperature of the surrounding environment. Also desirable about an oxygen sensor of this type is that it could be placed anywhere in the exhaust pipe, even at the cooler exit end, since the solid electrolyte of the sensor would not be dependent on the heat of the combustion gases for heat.
Sensor galvanic output voltage is dependent on temperature, as evidenced by the above recitation of the Nernst equation. Temperatures of the combustion gases from an internal combustion engine vary widely during operation, up to about a few hundred degrees Centigrade. Generally, these temperature variations after engine warm up are not detrimental to satisfactory operation of the typical oxygen sensor in use today. Currently, the oxygen sensors are employed in the exhaust gas system of an internal combustion engine to determine qualitatively whether the engine is operating at either of two conditions: (1) a fuel rich or (2) a fuel lean condition, as compared to stoichiometry. After equilibration, the exhaust gases from these two operating conditions have two widely different oxygen partial pressures, varying greatly in magnitude. At these conditions, the output voltage, as determined by the Nernst equation, is greatly dependent on the oxygen partial pressure ratios, yet relatively independent of temperature. Therefore, the typical oxygen sensor in use today does not require strict temperature control during operation.
However, for various reasons, it may be desired to operate internal combustion engines exclusively within lean combustion conditions, i.e., air-to-fuel ratios between 15:1 and 25:1, where changes in the after-combustion oxygen partial pressures are only gradual and slight. At these conditions, the output voltage is greatly dependent on temperature. Temperature control is critical for successful sensing of the exhaust emissions. The stoichiometric oxygen sensors commonly used today are inadequate for these purposes. Also, internal combustion engines operating exclusively within lean combustion conditions operate at cooler temperatures which may not be sufficient to adequately heat the solid electrolyte of the conventional oxygen sensor to its specified operating temperature.
Many heated oxygen sensors have been previously proposed in the art. These prior heated oxygen sensors generally comprise an elongated ceramic heater which positively heats the solid electrolyte body of the sensor. The heater element is typically inserted into an elongated cylindrical hole formed in the solid electrolyte body. An example of a prior heated oxygen sensor of this type is U.S. Pat. No. 4,578,174 to Kato et al.
For automotive applications particularly, a heated oxygen sensor should be rugged, reliable, and readily manufacturable at a low cost. It is also desirable that the heater components be readily adaptable to the current oxygen sensor design and manufacturing techniques. The present invention describes a novel concept for packaging a heated oxygen sensor with a ceramic rod heater. This heated oxygen sensor is easy to fabricate, can be built at minimal cost, and provides a rugged, reliable sensor assembly. It involves initially forming a self aligning heater subassembly comprising an elongated ceramic heater and a gripping body. The heater subassembly is also ruged and reliable, yet simple and amenable to mass production. Further the subassembly is Airedale incorporated into conventional unheated oxygen sensors typified by the above mentioned U.S. Pat. No. 3,844,920 to Burgett et al.